Operators of industrial plants — whether chemical-processing, oil-and-gas or pulp-and-paper operations — face tough choices when it comes to repairing or replacing heat exchangers. Engineering departments must cope with ever-increasing production demands that push the limits of aging but critical equipment. At the same time, they are tasked with finding ways to improve operating efficiencies and reduce costs.
At times, lack of capital eliminates or delays the option of replacement. In those cases, plant personnel may be challenged with heat exchanger problems that cause unplanned downtime or demand escalating maintenance. Furthermore, issues such as fouling, fogging and corrosion can contribute to poor heat transfer, disrupt normal operations and often delay production schedules. If these and other challenges become so severe that the only viable option is replacement, engineers still must overcome other restrictions. Limited floor space, poor access for equipment installation and tight department budgets further constrain the number of viable solutions available.
Fortunately, a growing number of innovative options exist when it comes to augmenting heat transfer in applications with low heat transfer coefficients. Tubular inserts are one prospective solution for heat exchangers. These insert systems, which can be tailored for a particular operating environment, can be retrofit for an existing system or engineered into a new unit. Possible designs include rotating or vibrating helical inserts and wire-matrix systems (table 1).
This article illustrates how wire-mesh inserts improved one application with laminar flow. It also explains how tubular inserts can improve heat transfer in industrial process heat exchangers.
TABLE 1. Tubular inserts are one prospective solution for heat exchangers. Possible insert designs include rotating or vibrating helical inserts as well as wire-matrix systems.
Pulp Mill: Inserts Overcome Space Limitations
If processing plant budgets and floor space were unlimited, engineers might simply design an enormous turbulent-flow heat exchanger — without regard to tube length or diameter — to accommodate demanding operating parameters. That is clearly not the typical scenario in today’s world, where everyone from senior management to maintenance supervisors are asked to provide more in tighter windows and use fewer resources in the process.
Take, for example, the case of a heat transfer challenge faced by a pulp mill in northern Alberta, Canada. Black-liquor coolers for lignin applications typically are designed to operate with low viscosity liquors, so turbulent-flow heat exchangers are impractical. But, for this particular application, the black-liquor strength was up to 38 percent, with a viscosity at temperature of 8 cP. In addition, pressure-drop restrictions were tight for the unit — under 6.5 kPa. A turbulent-flow heat exchanger was impractical due to these parameters and the limited footprint available for the equipment.
Initially, the laminar-flow heat exchanger designed included a 20”-diameter shell equipped with 343 tubes and no inserts. A larger design would require a bigger footprint within the facility as well as cost significantly more to fabricate. It also would be more complicated to install than a smaller, efficient unit that incorporated tube inserts.
Replacement tubular inserts for heat exchangers can be retrofit into existing exchangers.
In general, wire-matrix inserts are not appropriate for services that contain fiber because particulate can plug the tubing and restrict flow. In this process, however, the black liquor is filtered by a proprietary filter technology designed to remove fiber from the pulp-mill liquors to avoid contamination of the product lignin. As a result, the black liquor passing through the coolers did not contain any fiber.
The fiber-free black liquor allowed a redesign of the unit utilizing wire-matrix inserts. The resulting exchanger was 57 percent smaller. It had a 14” shell equipped with 148 tubes. Now in service for more than three years, the pulp mill has not experienced plugging or loss in exchanger capacity.
Optimizing Heat Transfer for the Operating Environment
Tubular inserts like the wire-matrix insert used at the pulp mill are not suitable for every situation. Such inserts are most effective for shell-and-tube heat exchanger applications with laminar flow (Reynolds number less than 2100) in the tubes. The Reynolds number — the ratio of inertial forces to viscous forces — is used to predict flow patterns in different fluid-flow situations.
Inserts usually perform best in environments with slow-moving viscous fluids and a low Reynolds number. Some inserts are useful in the transition range (2100 to 5000 Reynolds), but as viscosity decreases, they are not as effective. Inserts are less commonly used in turbulent flow — those applications with a Reynolds number greater than 5000 — because they provide a more modest benefit and result in significantly increased pressure drop. In such a flow regime, simply increasing the velocity and, hence, Reynolds number often is sufficient.
Tubular inserts such as the static-mixer elements in this heat exchanger facilitate fast, uniform heat transfer in highly viscous fluid applications. Photo: Kenics heat exchanger photo courtesy of National Oilwell Varco (NOV).
Heat exchanger design is about utilizing the available pressure drop most efficiently. Once reasonable criteria for pressure drop are established, several options exist for improving heat transfer. One possibility is installing multiple tube passes. In this scenario, the flow is forced through the exchanger in one direction, utilizing half of the tubes, before it is looped back on an opposite path through the other half of the tubing. This design causes the velocity to increase and the pressure drop to go up dramatically; however, this may be a viable approach to achieving turbulent flow in some applications.
Often, however, this is not the best use of available pressure drop — especially if turbulent flow is not achieved. Such a flow configuration provides only a marginal improvement in heat transfer for a large increase in pressure drop. In this situation, inserts may be able to achieve a greater increase in heat transfer for a given pressure drop than increasing the number of tube passes.
In appropriate operating environments, tubular inserts are able to reduce the size and costs associated with heat exchangers by enhancing heat transfer mechanically. These devices are designed to break up laminar boundary layers and provide bulk mixing between the center of the tube and wall. They do so by moving heat or energy to or from the fluid inside the tubes (rapidly heating or cooling it in the process).
Many insert products can be tailored to the application by changing the geometry. For example, wire-matrix and helical inserts can have the wire size and pitch selected to optimize performance. Twisted tape can have its pitch varied. Some inserts are available in higher alloys to help achieve acceptable lifetime in corrosive and other aggressive applications.
Inserts may be part of the tube (e.g., rifled tubing) or a separate device. Most non-integral inserts can be removed for replacement, cleaning or other maintenance needs. In some cases, these devices are simple in nature, but the application know-how — that is, the heat transfer and pressure drop calculations — may be proprietary.
Before contacting an insert supplier, heat exchanger fabricator or an engineering firm, it is helpful to gather the facts for your application. That includes assessing the performance of your unit, taking a careful look at viscosity versus temperature, and available pressure drop. Also, consider what hidden problems might be negatively impacting heat transfer just under the shell.
Dealing with Fouling and Fogging Problems
Fouling is sometimes the primary culprit leading to poor heat exchanger performance. Heat transfer suffers as fouling layers build up on the outer diameter of tubing and reduce thermal conductivity. A poor heat transfer coefficient on the inside of the tube can lead to inverse-solubility-type fouling due to higher temperatures on the inside of the tube wall.
Some mechanical inserts are designed to rotate and scrape the surface. Tube inserts also can promote mixing, generating eddies near the wall that increase shear stress. This mitigating effect, however, is secondary to the impact on fouling provided by controlling the tube-wall temperature.
Examples of fouling mitigation include reduction in coking in heavy crude oil and heavy organic materials, and controlling polymerization and tar formation in organic materials such as nitrobenzene. In inorganic streams containing inverse-solubility salts such as sodium sulfate, lower wall temperature also can reduce fouling.
Fogging, or fine-droplet formation, occurs when mass transfer cannot keep up with heat transfer in a heat exchanger. Although fogging is not a common problem, it can occur in certain condensing applications. Most commonly, those involve the recovery of high molecular-weight organics from noncondensable (inert) gas streams.
Where applicable, inserts can be used to increase the mass transfer coefficient in the same way as they increase the heat transfer coefficient. This can help to reduce or eliminate such problems.